Past and Future Solar Eclipses

Question:  I am interested in knowing how many solar eclipses have ever occurred in the past, and are likely to ever occur in the future based on widely accepted astronomical models.  Can you also provide dates for these events?  Thank you.  – Randy

Answer:  Listing the number of solar eclipses that have ever occurred is a bit of a tall order.  There are approximately 4 or 5 solar eclipses of various types (partial, annular, penumbral, total) per year, both in the past and in the future.  As for future solar eclipses, see the solar eclipse listing for the next 10 years.  This site also allows one to list solar eclipses which happened as far back as 1900.

Jeff Mangum

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What is the Critical Mass at Which a Star Becomes a Black Hole?

Question:  How do you calculate the critical mass for a star to become a black hole? I’m an undergraduate Math major, and this is my senior thesis and I really have no idea where to start.  – Lauren

Answer:  An earlier post to this blog on “Properties of Stars which Result in Black Holes” provides a general overview to this question.  Since the exact mass of an object like a star that must ultimately become a black hole is a function of its radius, there isn’t an exact mass above which that object must collapse to a black hole.  Said another way, any object which collapses to the point where its radius is less than a certain limit must ultimately become a black hole.  This radius is called the Schwarzschild radius (Rs), and it is given by the following equation:

Rs = 2MG/c^2

where M is the mass of the object, G is the gravitational constant, and c is the speed of light.  If you plug in values for the constants G and c and use solar masses for M and km for Rs, this equation reduces to the following rather simple form:

Rs = 2.95*M(solar masses) km

So, for a star with the same mass as our Sun, the Schwarzschild radius is about 3 km, or about 2 miles.  In general, stars with final masses in the range 2 to 3 solar masses are believed to ultimately collapse to a black hole.

Jeff Mangum

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Top Achievements for the NRAO Very Large Array

Question:  I’m embarrassed to ask this, as I feel after working here for so long I should already know the answer. But here it is: What are the top 3 discoveries/achievements of the VLA? And why?  — Doug

Answer:  As you can imagine, an actually ranking of the Very Large Array (VLA) achievements would be open to debate.  Kind of “beauty in the eye of the beholder” problem.  Dave Finley, NRAO Public Information Officer, provided the following links listing what NRAO believes are the VLA’s top scientific achievements:

Jeff Mangum

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Can Brain Waves be Detected by a Satellite Radio Telescope Array?

Question:  I have a question that pertains both to space and life on the ground.  It is my theory that, in essence, the U.S. communications and spy satellite network could be used as a massive (about 13 billion square-mile) radio telescope array which surrounds Earth that would have sufficient resolution and sensitivity to pick up the minute bioelectromagnetic signals given off by the human brain.  Could you comment on whether such a space-based radio telescope system as described would be able to pick up brain waves of us on Earth?  – John

Answer: Brain waves propagate at about 0.5 to 25 Hz.  You would need a radio telescope system that is capable of measuring these wavelengths, which are in fact quite low for radio telescopes.  Furthermore, as the ability to distinguish one radio source from another that is nearby is a function of the spatial resolution of a radio telescope, which is proportional to the wavelength of observation divided by the average separation between the antenna array elements, the low brain wave frequency makes it effectively impossible to distinguish one source of brain waves from another.  Finally, I believe that there would be many sources of emission in the 0.5 to 25 Hz range from sources other than human brains, which would present a “confusion” problem for these measurements.

Jeff Mangum


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Why is our Sun Alone?

Question:  As per observations, most of the stars are born in clusters (i.e. in open clusters). Then why our sun is an isolated star with no companions?  – Vinod

Answer:  Our Sun was probably born in a group of stars, but later escaped from or simply fell-out of this group.  Our Sun may have been pushed away due to an encounter with another star or a much larger object, such as a molecular cloud.  Or, and this scenario is a bit more likely, our Sun simply didn’t keep pace with its birth mates.  Since it takes about 135 million years for our Sun to orbit our Galaxy, small differences in the rate at which our Sun and the stars that were born with it will spread out over the 5 billion year age of our Sun.  Stars travelling slightly slower than our Sun will get left behind, which those stars that are travelling slightly faster will leave our Sun in the dust (interstellar, that is).  In the end, the result is a dispersed group of stars who once shared a common beginning.

Jeff Mangum

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Are Quasars and Radio Galaxies the Same Thing?

Question:  I’m interested in the differences and similarities between radio galaxies and quasars. Different sources say different things. Are radio galaxies just quasars with the “jets” pointed away from us, or are there other differences between them?  – Steve

Answer:  It turns out that even though quasars, which is short for “quasi-stellar radio source”, were first discovered as sources of radio emission that appeared to be point-like (like a “star”) in the optical, only about 10% of the known quasars today are also sources of radio emission.  You can think of radio galaxies and quasars as just different kinds of galaxies.  There are lots of radio galaxies, some of which are also quasars.  There are also lots of quasars, only a fraction of which are also sources of radio emission.

Jeff Mangum


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Dark Energy, Universal Expansion, and Peculiar Galaxy Motions

Question:  I have been hearing lately in some space documentaries that black energy, which makes the universe expand and at an increasing speed, separate and repulse objects such as galaxies from one another, am I correct?  But i also heard that the Andromeda galaxy is heading straight for us and should merge with the milky way, isn’t it contradictory? Thank you in advance for taking the time. – David

Answer:  I suspect that what you mean by “black energy” is really Dark Energy, which is one of several explanations for the acceleration of the expansion of the Universe.  The folks at NASA have a very nice page which describes what we think Dark Energy is, which you should certainly check out.  You are correct that Dark Energy is theorized to act like a repulsive force which pushes galaxies apart.  As for the Andromeda galaxy’s peculiar motion toward us, this is due to the mutual gravitational pull between our galaxy and Andromeda.  This is a “small-scale” affect, while the overall expansion of the Universe happens on a larger scale.

Jeff Mangum


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Will Dark Energy Tear Apart Black Holes?

Question:  Will dark energy eventually tear apart black holes, especially supermassive black holes?  – Reinaldo

Answer:  Let’s start with a basic understanding what what Dark Energy is thought to be.  It is a hypothetical form of energy which is thought to pervade the universe.  It has been proposed as the source of the measured acceleration of the expansion of our universe.  As a physical entity it is really quite “thin”, having a density of about 10^(-29) grams per cubic centimeter, so its affect of things like planets, stars, humans, and black holes is exceedingly small.  But, because it is thought to fill the universe its affect on the expansion of the universe is very important.  So, to answer your question, Dark Energy does not affect the properties of black holes, so in fact cannot disrupt them in any way.

Jeff Mangum

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Have We Ever Received Radio Signals From Space?

Question:  Hello I was wondering have we ever received any radio signals from space? What I’m trying to ask has earth ever received a message from space?  – Brandon

Answer:  Well, in fact, all objects in the universe that emit electromagnetic radiation (or “light”) emit radio waves.  Remember that “radio waves” are just one small chunk of what physicists call the “electromagnetic spectrum”.  So, in a literal sense, the answer to your question is yes.  I believe, though, that what you are really asking is if we have received an artificial radio signal from another civilization.  The answer to that question is definitely no.  That indeed would an event that we would all hear about!

Jeff Mangum

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Star Gazer Lives On

Hi Folks,

Through a very interesting article in the latest edition of Astronomy Beat, produced by my colleagues at the Astronomical Society of the Pacific (ASP), I was re-introduced to a wonderful little PBS-produced television show called “Star Gazers”.  These 5-ish minute episodes describing celestial events viewable by backyard astronomers were hosted by a guy named Jack Horkheimer from 1976 through 2010.  Jack obtained a kind of cult following for these Star Gazer (actually, it was originally titled “Star Hustler”) episodes by his lovably eccentric yet comfortable style of describing the astronomy happening over our heads.  Jack’s episodes are posted to YouTube (like most everything else), so you can get a sample of those classic shows.

The modern version of Star Gazers (note the subtle change in the title from its predecessor, as there are now three hosts for this show) is fantastic!  I highly recommend it for those interested in finding out what astronomical events are taking place each week.  You don’t need to be a degree-carrying scientist to understand the information presented in these episodes either.  It is definitely a good use of five minutes each week!

Jeff Mangum


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